'were adept at' is a correct and usable phrase in written English.
It is used to refer to someone being skillful at something. For example, "The students were adept at understanding the new material quickly."
We were adept at it for centuries.
"Kids were adept at getting around the blocks," she said.
At the time, many of the business unit heads were adept at competing, but few were adept at collaborating.
They were adept at managing a ring of subordinates.
Partnerships were adept at influencing decision makers to create or rewrite policies and improve built environments.
Its managers were adept at managing supplier relationships to make certain everyone understood the company's goals.
Mr. Floyd said his members were adept at quelling combustible situations and defusing threats.
All were adept at the long lines and the rapid-fire ornamentation of bel canto style.
Despite one overgenerous account, in Time, the Americans were adept at little else but self-promotion.
They were adept at selling products from anywhere in the world.
The owners of the newspaper were adept at gathering political intelligence.
